QPP (Quiescent cell _.Proline di-_Peptidase), also termed DPP2/7, is a vesicular protease that cleaves di- peptides from the N-terminus of proteins, thereby impacting their half-life and/or receptor specificity. While the full functional profile of this enzyme is far from understood, in vitro experiments demonstrate that QPP is a major regulator of the Go quiescence program in primary lymphocytes and neuronal cells. The primary objective of this renewal application is to analyze the mechanism of QPP-mediated survival in resting lymphocytes. Aim I: The most direct approach to define the functional role of QPP is to generate a mutant mouse deficient in QPP. Since previous attempts to derive conventional QPP# mice were curbed by embryonic lethality, we propose a novel alternative to generate the mutant mouse, utilizing the Cre/IoxP recombinase system that allows conditional and inducible genome alterations. Aim I1" Inhibition of QPP advances quiescent lymphocytes into cell cycle, leading to upregulation of c- Myc and p53 and finally apoptosis induction. Thus, it is postulated that QPP enzyme activity is required for maintaining the Go program. To test this hypothesis, the control of expression and mechanism of action of QPP will be delineated. Special emphasis will be given to LKLF and STAT5, two transcription factors that are implicated in positive and negative regulation of QPP expression, respectively. Furthermore, the role of c-Myc and p53 in the apoptosis pathway will be analyzed. Aim II1: To fully understand the Go survival program in eukaryotic cells, it is essential to define the physiological substrate(s) of QPP. Since this is the most ambitious project, various approaches will be used to reach this goal, including the screen of combinatorial dipeptide substrate libraries, the structural comparison of human and murine QPP with that of DPP4, and the isolation of a physiological substrate(s) by affinity matrix chromatography. Collectively, these studies will augment our limited understanding of the constitutive Go survival program in eukaryotic cells.